G-d promised the People of Israel that there will always be a number of Jews somewhere in the world who observe the Shabbat (Sabbath) fully.

As we find recorded in the Torah, G-d promised the People of Israel that there will always be a number of Jews somewhere in the world who observe the Shabbat (Sabbath) fully, as prescribed by the Torah. The Shabbat is the sign of an everlasting pact between G-d and the Jewish People, as we find in the Torah:
And the children of Israel shall observe the Sabbath (Exodus 31:16-17).
Commenting on this verse, the Sages interpret this is as a promise the Jewish People will never abandon the observance of the Sabbath altogether. At least a segment of the nation will always “remember the Sabbath day, to keep it holy,” as G-d commanded.
This promise is all the more surprising when viewed in the light of Jewish history, particularly in the Diaspora. Again and again, we find that when our adversaries attempted to entice us to abandon the Torah and embrace their culture and religion, their first target was Shabbat.
This makes it all the more amazing that to this very day, every Friday evening, as the sun sinks into the western sky, the Shabbat is welcomed warmly by Orthodox Jews the world over. Despite the extreme travails of the exile, there remains a solid core of Orthodox Jews who observe the Shabbat fully, just as their forefathers did thousands of years ago.
All the attempts of the gentiles to sever the sacred bond of the Jewish People with the “most precious of days” – the Shabbat – have come to naught. Some used brute, barbaric force. Others armed themselves with taunts, persecution and humiliating decrees against the People of Israel and their Sabbath. The Greeks and the Romans courted the favor of the masses with barbaric “entertainment.” In their eyes, this took the form of cruel torment of the those who persisted in honoring the sanctity of the Seventh Day. The idea of “wasting” a full seventh of the world's human resources by refraining from productive work one full day each week was preposterous in the eyes of the ambitious Greeks and Romans.
It was only much later that the gentile world acknowledged the benefit of a weekly day of rest. Even then, it was a far cry from the Jewish Sabbath, a day of spiritual renewal. For the gentiles, Sunday or Friday, or whatever day they might choose, is a day of physical rest and repose rather than an opportunity to draw nearer to the Creator. Nonetheless, despite the torment and persecutions, the Jewish People guarded and preserved their bond with Him who gave them the Sabbath and refused to profane it with even the slightest violation of its laws and regulations.
G-d established this pact of the Shabbat with the People of Israel and with them alone: “… between Me and the Children of Israel, it shall be an everlasting sign...”
This covenant was established between the Creator of the universe and His people, Israel, alone. The Sages commented on the words “between Me and the Children of Israel...” as follows: “And not between Me and the (gentile) nations.”
This unique bond of Israel with the Sabbath is reflected in the Sabbath prayers:
“...and You did not bestow it, O L-rd, our G-d, upon the nations of the lands, and You did not bequeath it, our King, to those who worship graven idols; the uncircumcised shall not abide in its repose, for it is to Your people Israel that You bequeathed it with love, to the seed of Jacob, whom You have chosen.” (from the prayer for Shabbat morning)
This promise that there would always be a special bond between Israel and the Creator, in the form of the Shabbat, has been miraculously fulfilled against all odds. As we know, the early Christians also observed Saturday as their Sabbath. Heaven orchestrated events in such a way that, when the new religion began to spread and win additional adherents, the church decided to transfer its day of rest to Sunday. As a result, the sign of Israel's pact with G-d remained unique and incontestable.
Similarly, with the rise of Mohammedanism; rather than sharing a day of rest with the Jews, the Muslims kept their day of rest on Fridays and as it remains to this day.
Only Israel continues to mark its day of rest on that day of the week originally designated by the Creator. Again each week, the People of Israel confirm their exclusive right to the sign of their unique pact with their Creator.